1. Field of the Invention
This invention relates to devices which dispense active agents to an environment of use at a controlled and continuous rate over a prolonged period of time. In a preferred embodiment, it relates to delivery devices for the sustained release of an agent to a substantially constant pH environment, such as an agent to a mammalian patient, and the like.
2. The Prior Art
The use of poly(carboxylic acids) as enteric coatings has been reported by Lappas and McKeehan at 51 J. Pharm, Sci. 808 (1962), at 54 J. Pharm. Sci. 176 (1965) and at 56 J. Pharm. Sci. 1257 (1967).
As is well known, enteric coatings are special coatings applied to ingestible tablets or capsules which prevent release and absorption of their contents until the tablets reach the intestines.
The poly(carboxylic acids) are well suited to this application and the widely varying pH conditions of the gastrointestinal tract. In the highly acidic stomach (pH 2) poly(carboxylic acids) are present completely as unionized hydrophobic species which are water insoluble and which prevent the release of any enclosed drug. As the poly(carboxylic acids) move on to the intestine, they are exposed to alkaline conditions (pH of up to 9) in which they ionize to soluble hydrophilic species and release the enclosed drug.
With these prior enteric coating teachings, the release of drug is merely delayed. The release is essentially a pH-dependent step fuction.
There is no release of drug in the acidic stomach; there is release of all the drug as the encapsulated drug enters the intestine and the pH of the environment changes to an alkaline value.